1. Field of the Invention
The present invention relates to a satellite communication system comprising a plurality of x limited scan spot beam ports and, more particularly, to a satellite communication system comprising a plurality of x limited scan spot beam ports and a plurality of N transponders, where x.gtoreq.N and each spot beam port is scanned over a separate limited portion of the overall region to be serviced by the satellite and each transponder is associated with one or more ports and can only be coupled to one port at a time so that each transponder handles a substantially equal amount of traffic.
2. Description of the Prior Art
The early satellite communication system designs employed an area coverage beam which provided interconnections on either a time-division multiple access (TDMA) basis or a frequency-division multiple access (FDMA) basis. Such designs had the disadvantage of low antenna gain, and frequency reuse was only possible by the use of polarization techniques. More recent designs use (a) multiple narrow-angle fixed spot beams with on-board satellite switching to provide frequency reuse, high capacity, and high antenna gain, (b) a single scanning beam to provide high antenna gain, (c) the combination of an area coverage beam and multiple narrow-angle fixed spot beams to provide high capacity, and (d) the combination of multiple narrow-angle fixed spot beams and a single scanning beam with on-board satellite switching.
A typical prior art design is shown in U.S. Pat. No. 3,711,855, issued to W. G. Schmidt et al on Jan. 16, 1973, which illustrates a conventional multiple-transponder satellite with n transponders for n or more ground stations where each transponder covers a particular portion of the frequency spectrum and no two ground stations may concurrently transmit in the same frequency band. Another design is shown in U.S. Pat. No. 3,928,804, issued to W. G. Schmidt et al on Dec. 23, 1975, where a plurality of receive spotbeam antennas are selectively connected to a plurality of transmit spotbeam antennas by an on-board switching matrix. Additionally, several other separate receive and transmit spotbeam antennas are connected to a common receiver and transmitter, respectively, by a respective on-board input and output switch.
in the more recently proposed Intelsat V satellite communication system, it has been proposed to concurrently use an overlapping fixed spot beam, a larger area zone coverage beam, and a still larger hemisphere coverage beam, each type of beam using a different frequency spectrum and/or polarization for transmitting its signals to avoid interference at receivers capable of receiving signals in more than one type of beam, for increasing satellite throughput capacity. Additionally, the concurrent use of both a plurality of fixed spot beams and an area coverage beam, which overlaps the spot beams and where all beams use the same frequency spectrum for transmission, has been proposed in the article "Spectral Reuse in 12 GHz Satellite Communication Systems" by D. O. Reudink et al in IEEE Conference Record ICC 77, June 12-15, 1977, Chicago, Ill., Vol. 3 at pp. 37.5-32 to 37.5-35. There, arrangements are disclosed for substantially cancelling the interference of the received signals originally transmitted in the area coverage beam with the desired spot beam signals received in each of the overlapped spot beam receiver areas.
Arrangements for using a movable beam in satellite, airborne, or mobile communication systems have also been disclosed. For example, U.S. Pat. No. 3,750,175 issued to R. M. Lockerd et al on July 31, 1973 discloses a modular electronics communication system comprising a plurality of radiating elements formed into an antenna array for transmitting and receiving communication frequency signals and employing a central processor to generate the transmitted signals and process the received frequencies through a manifold arrangement. Each radiating element connects to the manifold through a module made up of integrated microwave circuitry including a mixer coupled to a local oscillator and a phase shifter coupled to a beam steering computer. By means of the beam steering computer the antenna can be made to scan various preselected areas to primarily overcome tolerances in the satellite's or aircraft's attitude control system and maintain a beam at a desired target area. Additionally, the possible use of steerable beams and time-hopped steerable beams was suggested, but no implementation thereof shown, in Progress in Astronautics and Aeronautics, Vol. 33, pp. 503-531 at page 507 in the article "Characteristics and Applications of Multibeam Spacecraft Antennas", which was presented as Paper 72-530 at the AIAA 4th Communications Satellite Systems Conference, Washington, D.C., Apr. 24-26, 1972.
In the article "A Scanning Spot Beam Satellite System" by D. O. Reudink et al in The Bell System Technical Journal, Vol. 56, No. 8, October 1977 at pp. 1549-1560 a satellite communication system was disclosed which involves the use of a steerable spot beam which can be rapidly scanned across the entire service region via a phased array antenna, thereby providing universal coverage. When used in conjunction with a multitude of fixed spot beams, the resulting hybrid system has the advantage of frequency reuse, high antenna gain, and identical transponders. However, such a hybrid system does not utilize the transponders efficiently because of nonuniform traffic demands from the various ground areas covered.
In an Article "Efficient Utilization of Satellite Transponders Via Time-Division Multibeam Scanning" by A. Acampora et al in The Bell System Technical Journal, Vol. 57, No. 8, October 1978, a space segment of a satellite system is disclosed wherein a fixed number of identical transponders are shared among a larger number of spot beam regions which completely span a large total service area. In the arrangement, time-division multiple access techniques are employed and each transponder is rapidly scanned over appropriately defined group pairs of spot beam regions, thereby establishing full coverage and full interconnectivity. In the Acampora et al arrangement, the resources of the transponders can be allocated as needed on a time division basis among a far larger number of spot beam footprints which completely cover the entire service region. For such a system, the capacity required by each footprint must be smaller than the capacity of a single transponder since two or more transponders cannot simultaneously serve the same footprint on a noninterfering basis. Naturally, the total capacity required from all regions must be less than the total capacity of all transponders. A powerful result proven in the Acampora et al article is that a TDMA traffic assignment can always be found provided that the global properties of the traffic matrix satisfy only these broad requirements, i.e., the existence of an assignment does not depend upon the details of beam-to-beam traffic entities. For realistic traffic matrices, the assignment can usually be made to avoid interference caused by the antenna sidelobes of neighboring beams.
A practical problem associated with this approach is the need to provide rapid interconnection between any satellite transponder and any service region. This can be done either with an N.times.M high power crossbar switch to connect any of the N transponders to any of the M footprints, or with a phased array antenna capable of simultaneously forming N independent beams, directed anywhere within the service region. This latter approach always requires a large number of phase shifters which can be equal to the number of transponders multiplied by the number of antenna elements. In addition, the N independent signals must be combined prior to each element, either in power or in voltage. Voltage addition is simpler, but the combined signal must then be power amplified with a resultant intermodulation power loss.
The problem remaining in the prior art is to provide a satellite communication system which is capable of high transponder utilization efficiency and overcomes the shortcomings indicated with the above-described systems while providing simplified on-board electronics.